This disclosure pertains to magnetic shielding of an enclosure such as a room containing equipment that is sensitive to the effects of external magnetic fields, and to enclosures shielded in such a manner. Exemplary field-sensitive systems that can be contained in such a magnetically shielded enclosure include systems for performing microlithography using a charged particle beam.
The growth of modern processing and analytical technology has included more extensive use of techniques that employ a charged particle beam (e.g., electron beam or ion beam). Accompanying more extensive use of these techniques generally has been a demand for progressively more accurate and precise performance from the systems that perform the techniques. For example, increasingly greater image resolution is being demanded from electron microscopy. Also, increasingly greater pattern-transfer resolution and accuracy is being demanded from charged-particle-beam (CPB) microlithography, which is a key xe2x80x9cnext-generation lithographyxe2x80x9d technology being actively developed for fabricating microelectronic devices.
Obtaining greater pattern-transfer accuracy from a CPB microlithography system requires application of more stringent measures to prevent the charged particle beam from being influenced uncontrollably by stray external and internal magnetic and electrical fields. External stray magnetic fields include magnetic fields produced by the earth, events in outer space, and by nearby man-made equipment such as power equipment, power cables, and elevators, for example. Similarly, external stray electrical fields can be produced by any of various sources, both natural and man-made. xe2x80x9cInternalxe2x80x9d stray fields usually are produced by components of the CPB microlithography system located, for example, inside the xe2x80x9clens columnxe2x80x9d (vacuum chamber that houses the CPB optical system) and/or inside the xe2x80x9csubstrate chamberxe2x80x9d (vacuum chamber that houses the substrate stage and peripheral components). Even if the magnitude or fluctuation amplitude of a stray field is very small, the field nevertheless can cause an undesired change in the trajectory and/or position of the charged particle beam sufficient to destroy any prospect of achieving a desired accuracy and precision of pattern transfer. For example, if the charged particle beam is being used to transfer a pattern having linewidths of, e.g., 70 nm, the importance of reducing the effect of a stray magnetic and/or electrical field, even an extremely small-magnitude field, on the beam is readily appreciated.
As noted above, potentially troublesome fields can be electrical or magnetic, static or dynamic (fluctuating), strong or weak, man-made or natural, internal or external. An example of an internal field is a field generated by a component of the system, such as a stray magnetic field produced by an electron lens or deflector or by a reticle stage or substrate stage. An example of an external field is a field produced by the earth or by nearby industrial activity.
For shielding purposes, conventional CPB microlithography systems usually include one or more magnetic shields located inside the lens column and inside the substrate chamber. For example, shielding may be associated with certain peripheral components located in or near these chambers, such as wafer loaders, reticle loaders, electromagnetic lenses, stage motors, vacuum pumps, etc. Another conventional manner of shielding CPB microlithography lens columns and substrate chambers is the application to the chambers of a single, double, or triple coating of a material having a high initial magnetic permeability such as Permalloy. Alternatively or in addition, the lens columns and substrate chambers themselves are made of a material having high initial magnetic permeability, such as Permalloy.
Reference is made to FIG. 6 that schematically depicts exemplary conventional magnetic shielding used in association with a CPB microlithography system. The subject system 100 comprises an electron gun 1 that generates an electron beam that propagates in a downstream direction (downward in the figure). A substrate stage 24 includes a xe2x80x9cwafer chuckxe2x80x9d on which the lithographic substrate is mounted for exposure by the electron beam. The electron gun 1 and an electron-optical system (not detailed) extending along the trajectory of the electron beam are contained in a xe2x80x9clens columnxe2x80x9d 31, and the substrate stage 24 is contained in a substrate chamber 33. The lens column 31, typically made of invar or soft iron, is connected via a duct 37 to a vacuum pump (not shown). The substrate chamber 33 typically is made of aluminum or non-magnetic stainless steel. The lens column 31 and substrate chamber 33 are conjoined and thus communicate with each other, allowing their respective internal spaces to be shared. Although not detailed, the microlithography system 100 includes one or more condenser lenses that direct the electron beam onto a reticle, one or more beam-trimming apertures, a reticle stage, a projection-lens assembly that demagnifies and projects the electron beam (propagating downstream of the reticle) onto a lithographic substrate, and one or more beam deflectors for beam positioning and aberration correction.
The microlithography system 100 is enclosed within a shielded enclosure 21. The enclosure 21 is effectively a chamber made of a material having high initial magnetic permeability. The enclosure 21 houses the entire lens column 31 and substrate chamber 33 of the system 100.
The enclosure 21 defines various openings. For example, a vacuum duct 37 extends through the wall of the enclosure 21 to allow evacuation of the lens column 31 and substrate chamber 33. Other openings 22 in the enclosure 21 correspond with respective feed-through apertures 39 in the lens column 31 to allow passage of wires and the like to and from the lens column 31. Another opening (not shown) corresponds with a respective opening in the lens column 31 through which the reticle is moved to and from the reticle stage. Although not detailed in FIG. 6, yet another opening in the enclosure 21 allows passage of the lithographic substrate through a respective opening in the substrate chamber 33 through which the substrate is moved to and from the substrate stage 24. In addition, a gap 40 usually is associated with the location of a connecting member coupling the lens column 31 to the substrate chamber 33. Since the enclosure 21 typically is not unitary, other gaps in the enclosure 21 typically are provided at respective conjunctions of shield segments.
Openings and gaps in the magnetically permeable material of the enclosure 21 usually reduce the magnetic-shielding performance of the enclosure, sometimes to a level at which the shielding effect is inadequate. Consequently, an aperture or gap is provided in the enclosure usually only when necessary. To offset the consequences of providing apertures and gaps in the enclosure 21, it frequently is necessary to shield the walls of the room containing the enclosure 21 (with the CPB microlithography system 100 or other field-sensitive system inside). The shielded room can be xe2x80x9cpassivelyxe2x80x9d shielded, wherein the room walls simply are covered with a magnetic-shielding material. Alternatively or in addition, the room can be xe2x80x9cactivelyxe2x80x9d shielded, wherein the room walls include respective coils that generate respective magnetic fields extending usually in a selected direction normal to the plane of the wall. The coil-containing walls typically are separated by a distance from the system enclosed in the room (e.g., separated from the lens column and substrate chamber). By appropriate energization of one or more of the coils, a portion of an external magnetic field leaking into the room is cancelled by a countervailing magnetic field generated from by the coil(s). This technique is termed xe2x80x9cactive cancellation,xe2x80x9d and the coils are termed xe2x80x9cactive cancellers.xe2x80x9d
A conventional shielded room 81 including active cancellers is shown in FIG. 7, comprising three active cancellers each comprising a respective pair 83 and 83xe2x80x2, 85 and 85xe2x80x2, 87 and 87xe2x80x2 of opposing coils collectively arranged three-dimensionally. The coils can be situated on the inside or outside of the walls of the room 81. The arrows associated with the coils in the figure denote the respective directions of electric currents that flow in the coils. As a result of such current flow in the respective coils, the three pairs 83 and 83xe2x80x2, 85 and 85xe2x80x2, 87 and 87xe2x80x2 of coils generate respective magnetic fields in mutually perpendicular directions. The magnitude and direction of the electrical currents applied to respective pairs of coils can be adjusted as required to create, inside the room 81, a net magnetic field having a magnitude and direction that serve to cancel at least a portion of a stray external magnetic field penetrating into the room.
In a shielded room 81 configured as shown in FIG. 7, the best field-cancellation performance generally is obtained at or near the center of the room. As a result, despite energization of the active cancellers 83 and 83xe2x80x2, 85 and 85xe2x80x2, 87 and 87xe2x80x2, some stray magnetic flux leaking into the room 81 from outside tends to remain not canceled inside the room, especially near the wall surfaces. If a portion of a field-sensitive system is situated near a wall under such a condition, the residual non-canceled magnetic flux leaking through the respective wall into the room 81 penetrates into the system and adversely affects system performance. Preventing this effect conventionally requires that the system be situated at the center of a room 81 that is many times larger than the system. If the system is large, then the room 81 must be extremely large, which may be impractical or impossible to construct from the standpoint of cost and/or ability of the site to accommodate such a large room. Also, the maximum size of a wall member made of a shielding material such as Permalloy is limited. When constructing a shielded room, the complexity of joints between walls and the processing required to configure such joints increase greatly with increased room size. Furthermore, in larger active cancellers, the increased size of the coils and of the equipment required to power them can be serious problems.
In view of the shortcomings of the prior art as summarized above, the present invention provides, inter alia, magnetically shielded enclosures (e.g., rooms) and magnetic-shielding methods that produce a desired more complete shielding effect without having to make the enclosure prohibitively large. Also provided are magnetic-field-sensitive systems (e.g., charged-particle-beam lithography systems) enclosed in such enclosures.
According to a first aspect of the invention, magnetically shielded enclosures are provided for containing and magnetically shielding a field-sensitive system. (The field-sensitive system includes an internal shielding barrier substantially surrounding the system.) An embodiment of such an enclosure comprises multiple walls made of a material including a magnetically permeable material. The walls are configured relative to each other so as to define an internal magnetically shielded space that encloses the internal shielding barrier and thus the system. The enclosure embodiment also includes an aperture defined in at least one of the walls. The aperture is configured relative to a profile of the internal shielding barrier as shadowed on the apertured wall such that a first portion of an external magnetic flux leaking through the aperture into the internal magnetically shielded space at least partially cancels a second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space. The cancellation serves to reduce a net external magnetic flux incident on the system.
The aperture can be configured to have a ring shape substantially surrounding the profile of the internal shielding barrier as shadowed on the apertured wall. The aperture desirably has a diameter (or other cross dimension) greater than the corresponding width of the profile of the internal shielding barrier.
The apertured wall further can comprise at least one thickening member extending around an edge of the aperture. The thickening member serves to increase the thickness of magnetically permeable material adjacent the aperture, relative to the thickness of the magnetically permeable material in the apertured wall.
The aperture can be surrounded by a peripheral region defined from the apertured wall. In this configuration the peripheral region desirably comprises a magnetically permeable material having a magnetic permeability that is greater than the magnetic permeability of the magnetically permeable material of the wall.
According to another aspect of the invention, magnetically shielded enclosures are provided for containing and magnetically shielding a field-sensitive system. An embodiment of such an enclosure comprises an internal shielding barrier substantially surrounding the system, wherein the internal shielding barrier is made of a material including a magnetically permeable material. The enclosure also includes an outer magnetically shielded enclosure substantially surrounding the internal shielding barrier. The outer enclosure comprises: (a) multiple walls made of a material including a magnetically permeable material, wherein the walls are configured relative to each other so as to define an internal magnetically shielded space containing the internal shielding barrier, and (b) an aperture defined in at least one of the walls. The aperture is configured relative to a profile of the internal shielding barrier as shadowed on the apertured wall such that a first portion of an external magnetic flux leaking through the aperture into the internal magnetically shielded space at least partially cancels a second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space. The cancellation serves to reduce a net external magnetic flux incident on the system.
In this enclosure the field-sensitive system can be, for example, a charged-particle-beam microlithography system. With such a system, the internal shielding barrier can be configured collectively as a lens column and substrate chamber of the system.
According to another aspect of the invention, methods are provided for magnetically shielding a field-sensitive system substantially surrounded by an internal shielding barrier. In an embodiment of such a method one step involves configuring multiple walls so as to define an internal magnetically shielded space, wherein each wall is made of a material including a magnetically permeable material. Another step involves situating the system, surrounded by the internal shielding barrier, in the internal magnetically shielded space. Another step involves defining an aperture in at least one of the walls, wherein the aperture is configured relative to a profile of the internal shielding barrier as shadowed on the apertured wall such that a first portion of an external magnetic flux leaking through the aperture into the internal magnetically shielded space at least partially cancels a second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space. The cancellation serves to reduce a net external magnetic flux incident on the system.
The step of defining the aperture can comprise configuring the aperture to have a ring shape substantially surrounding the profile of the internal shielding barrier as shadowed on the apertured wall. The diameter (or other cross dimension) of the aperture can be greater than the diameter or other cross-dimension of the internal shielding barrier.
The step of defining the aperture further can comprise extending at least one thickening member around an edge of the aperture, so as to increase the thickness of magnetically permeable material adjacent the aperture, relative to the thickness of the magnetically permeable material in the apertured wall. Alternatively, the aperture can be surrounded by a peripheral region of the apertured wall. In this alternative configuration the step of defining the aperture further can comprise providing the peripheral region with a magnetically permeable material having a magnetic permeability greater than the magnetic permeability of the wall material. In either of these configurations, magnetic flux entering the thickened or peripheral region from the aperture increase the proportion of the leaking flux directed toward the wall of the enclosure rather than toward the internal shielding barrier. In addition to or alternatively to performing a field-canceling role, the aperture can be situated and configured such that a combination of the first portion of the external magnetic flux leaking through the aperture into the internal magnetically shielded space and the second portion of the external magnetic flux leaking through the wall into the internal magnetically shielded space collectively are oriented in direction(s) in which the leaked flux penetrates the internal shielding barrier only poorly at best. This achieves the result of minimizing the stray magnetic flux that penetrates the internal shielding barrier to the system.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.